(1) Field of the Invention
The present invention relates to an optical recording material of the type in which a film of a metal, an organic coloring material or the like is formed on a substrate, the film is irradiated with laser beams, pits (holes) are formed in irradiated areas or a deformation or a phase change is caused in irradiated areas, and information is recorded and reproduced by utilizing a change of the reflectance in these areas.
(2) Description of the Related Art
In an optical recording medium of the pit-forming type, the recording film is irradiated with a laser beam modulated with an information signal, and the recording film is locally melted and evaporated by the thermal action of the laser beam to form a row of pits corresponding to the information signal. Since the reflectance of the recorded area wherein the row pits is thus formed is much different from the reflectance of the area wherein pits are not formed, reproduction is effected by utilizing this change of the reflectance.
Currently, a semiconductor laser is used as the laser light source for diminishing the size of the recording apparatus. The semiconductor laser is advantageous in that its size is small, its price is moderate and it is easily modulated, but its output is small and the short-wavelength oscillation required for high-density recording is impossible. Accordingly, to perform high-speed recording on an optical recording medium by using a small-size recording apparatus having a semiconductor laser mounted thereon, the recording medium must be a recording film having high sensitivity, i.e., a recording film on which pits having a good shape can be formed with a low power laser. To form such a recording film, the material of the recording film must have a low melting point and a low heat conductivity, and show an appropriate light absorption and an appropriate light reflectance at the oscillation wavelength (780 to 850 nm) of the semiconductor laser. In addition to these properties of the recording film, the surface tension of the melt of the recording film and the surface tension of the substrate on which the recording film is formed have a strong influence on the formation of pits. Accordingly, even if the same recording film is formed, the energy necessary for the formation of pits differs in accordance with the properties of the surface of the substrate on which the recording film is formed, and thus, the recording density or the shape the pits is often changed.
As the material of the recording film having properties required for attaining the above-mentioned recording characteristics, there can be mentioned films formed by vacuum evaporation deposition or sputtering of metals such as In, Ge, Sn, Pb, As, Sb, Bi, S, Se and Te, alloys thereof and composites of these metals with organic substances and oxides, and films formed by spin coating of organic coloring substances having a light absorption band in an oscillation wavelength region (780 to 850 nm) of the semiconductor laser.
Especially, films containing Sn as the main component, such as films of Sn.sub.x S.sub.100-x, Sn.sub.x Bi.sub.100-x and Sn.sub.x Se.sub.100-x (0&lt;x&lt;100) can be optical recording films having a high sensitivity because the melting point is low and the light absorption and reflectance at oscillation wavelengths (780 to 850 nm) of a semiconductor laser are high. Moreover, the above-mentioned films comprising tin as the main component are stable against oxidation and have a strong durability. Furthermore, if layers of a passive state-forming metal are formed contiguously to both sides of the above-mentioned film comprising tin as the main component, these layers act as stable oxide protecting films, and the stability against the oxidation of the film comprising tin as the main component is further improved, and a formation of pinholes by pitting is controlled, with the result that any increase of the pit error rate with the lapse of time is controlled. Especially when Sn.sub.x Se.sub.100-x (0&lt;x&lt;100) is used as the film substance comprising tin as the main component, an optical recording film having a high C/N ratio can be obtained and in which pits having a good shape can be formed.
In optical recording media of this type, the data transfer speed is low; about 1/2 of the data transfer speed in magnetic recording media such as magnetic tapes or magnetic disks. To eliminate this disadvantage, attempts have been made to increase the disk rotation speed to about 2,400 to about 3,600 rpm. To perform the recording at such a high speed while using a semiconductor laser having a low output, to reduce the price and size of the driving device for the optical recording medium, the sensitivity of the recording film must be further increased.
Attempts have been made to improve the recording sensitivity of the optical recording medium by forming a primer layer between the disk substrate and the recording film. As the primer layer, there are formed plasma films, sputtered films, vacuum evaporation deposition films, and spin coated films of various organic substances or organic substances containing a metal having a low melting point, and sputtered films of thermally decomposable metal oxides, nitrides or sulfides. As the organic substance used for formation of the above-mentioned primer layer, there are known fluorine-containing compounds such as hexafluoropropylene and tetrafluoroethylene, nitrogen-containing compounds such as acrylonitrile and propylamide, sulfur-containing compounds such as carbon disulfide, silicon-containing compounds such as hexamethyldisilane, saturated hydrocarbons such as methane and ethane, unsaturated hydrocarbons such as ethylene and acetylene, aromatic hydrocarbons such as benzene and styrene, and purine derivatives such as guanine and guanosine. Nevertheless, even in the optical recording medium having the above-mentioned primer layer formed between the conventional recording film and substrate, the recording sensitivity is still not high enough to allow recording at a rotation speed as high as 2,400 to 3,600 rpm.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical recording medium comprising a substrate and, formed thereon, a recording film having layers of a passive state-forming metal or an oxide thereof formed contiguously to both sides of a layer containing at least tin and selenium, which has a recording sensitivity sufficient to allow recording at a high rotation speed (i.e., 2,400 to 3,600 rpm) while using a semiconductor laser having a small size and a small output.
In accordance with the present invention, there is provided an optical recording medium of the type whereby information is recorded and reproduced by utilizing a change of the reflectance of an irradiated portion, caused by irradiating a recording film formed on a substrate with laser beams, said optical recording medium being characterized in that the recording film has a three-layer structure comprising a layer containing at least tin and selenium and layers of a passive state-forming metal or an oxide thereof, contiguous to both sides of said layer containing tin and selenium, and a film of a phthalocyanine compound having a light transmission of at least 80% at a wavelength of 770 to 850 nm is disposed between the recording film and the substrate.